1. Field of the Invention
The present invention relates to a boundary acoustic wave device for use as a resonator and a filter device, for example. In particular, the present invention relates to a boundary acoustic wave device in which at least a portion of an IDT electrode is embedded in a groove disposed in a piezoelectric substrate.
2. Description of the Related Art
The boundary acoustic wave device is an elastic wave device which does not require a package having a cavity and which can be miniaturized as compared with a surface acoustic wave device. Therefore, the boundary acoustic wave device has attracted rising attention in recent years.
For example, WO 2008/044411 A1 described below discloses a boundary acoustic wave device in which at least a portion of an IDT electrode is embedded in a groove disposed in a piezoelectric substrate. FIG. 78 is a magnified schematic cross-sectional view showing a portion of an IDT electrode of this boundary acoustic wave device. As shown in FIG. 78, a boundary acoustic wave device 101 is provided with a LiNbO3 substrate 102. A silicon oxide film 103 is disposed on the LiNbO3 substrate 102. An IDT electrode 104 is disposed at the boundary between the LiNbO3 substrate 102 and the silicon oxide film 103. The IDT electrode 104 is embedded in a groove 102a disposed in the LiNbO3 substrate 102. The upper surface of the IDT electrode 104 must be flush with the surface of the LiNbO3 substrate 102.
WO 2008/044411 A1 discloses that the upper surface of the IDT electrode 104 must be flush with the surface of the LiNbO3 substrate 102, the upper surface of the silicon oxide film 103 is flat and, thereby, a low insertion loss can be realized.
However, regarding the boundary acoustic wave device 101 in which the IDT electrode 104 is embedded in the groove 102a of the LiNbO3 substrate 102, there is a problem in that the acoustic velocity of propagating boundary acoustic wave cannot be increased sufficiently.